83 



their failing powers of multiplication by cell-division are com- 

 pletely restored. 



For the purpose of studying this process of conjugation in its 

 primitive simplicity we can hardly do better than take the minute 

 flagellate form Copromonas, which is found in water frequented 

 by frogs, from the excrement of which it derives its nutriment. 

 The adult organism (Fig. 37, A) consists of a very minute ovoid 

 mass of protoplasm with a single flagellum (tr.) springing from 

 the narrow end. Alongside the base of the flagellum is a definite 

 cell mouth (cytostome, cst.) through which solid particles of food 

 are taken into the interior of the body. Close to this there is 

 a contractile vacuole (CT.), accompanied by a "reservoir" (K) 

 into which it discharges. The nucleus (N) is situated nearer to 

 the broadly rounded hinder end of the body, which may also 

 contain a number of food-vacuoles (/#.). 



If the food supply be abundant the individual Copromonas will 

 grow and presently divide into two by simple longitudinal fission, 

 which commences at the narrow anterior end (Fig. 37, B D). 

 The division of the nucleus is said to be amitotic. The two 

 daughter cells separate, feed, grow and repeat the process, and in 

 this way a whole swarm of monads is produced. In the course of a 

 few days, however, they appear to become exhausted and conjuga- 

 tion sets in, the individuals uniting in pairs (Fig. 37, 2 5). The 

 result of each such union is a single larger individual, which may 

 either undergo a period of rest within the protection of a special 

 envelope or cyst (Fig. 37, 7), or at once assume the ordinary 

 form and begin to multiply with renewed activity (Fig. 37, V;. 

 For the continued existence of the species it is probably necessary 

 that the encysted monads should at some time or another be 

 swallowed by frogs and passed out again in the faeces, in order 

 that they may be brought in touch with the necessary food 

 supply. 



We have here, as in the case of Haematococcus described in 

 Chapter III., a perfectly typical example of conjugation 1 occurring 

 at longer or shorter intervals in the life cycle of the organism. The 

 whole process consists in the union of two separate cells, known 

 in this connection as gametes, to form a single cell known as the 

 zygote, and it is of the utmost importance to observe that not only 

 is there a union between the cytoplasm of the two gametes but 

 the nuclei also unite to form a single zygote nucleus. Indeed, 



1 Also known as syngamy or zygosis. 



G 2 



